The invention relates to a method and an apparatus for providing an image to be displayed on a screen, in particular on a TV screen or on a computer monitor, according to the preambles of claims 1, 12 and 16.
Such a method and apparatus are known in the art. However, in prior art it is also known that perspective deformations may occur when an image is displayed on a screen depending on the current position of a viewer watching the screen. That phenomenon shall now be explained in detail by referring to FIG. 3.
FIG. 3 is based on the assumption that a 2-dimensional image of a 3-dimensional scene is either taken by a camera, e.g. a TV camera or generated by a computer graphic program, e.g. a computer game. Moreover, an assumption is made about the location of the centre of the projection P and the rectangular viewport S of the original image, wherein P and S relate to the location where the original image is generated, e.g. a camera, but not necessarily to the location where it is later watched by a viewer. P and S are considered to form a fictive first pyramid as shown in FIG. 3. In the case that the image is taken with a camera, P is the optical centre of this camera and S is its light sensitive area. In the case that the image is generated by a computer graphic the parameters P and S can be considered as parameters of a virtual camera.
The original image might be generated by the camera or by the computer program by using different transformations known in the art:
One example for such a transformation is the change of the viewing point from which a particular scene is watched by the real or virtual camera.
Another example for such a transformation is the following one used in computer graphic applications for correcting texture mapping. Such a transformation may be described according to the following equation:                               [                                                    x                                                                    y                                              ]                =                              [                                                                                                      a                      ⁢                                              xe2x80x83                                            ⁢                      u                                        +                    bv                    +                    c                                                                                                                    du                    +                    ev                    +                    f                                                                        ]                    /                      (                          gu              +              hv              +              i                        )                                              (        1        )            
wherein:
the term (gu+hv+i) represents a division per pixel;
u,v are the co-ordinates of a pixel of the image before transformation;
x,y are the co-ordinates of the pixel of the image after the transformation; and
a,b,c,d,e,f,g,h and i are variable coefficients being individually defined by the graphic program.
However, irrespective as to whether the original image has been generated by conducting such transformations or not or as to whether the image has been generated by a camera or by a computer program, there is only one spatial position Q in the location where the image is later watched after its generation, i.e. in front of a screen 10 on which the image is displayed, from which a viewer can watch the image on the screen without any perspective deformations.
Said position Q is fix in relation to the position of the screen 10 and can be calculated form the above-mentioned parameters P and S according to a method known in the art.
The position Q is illustrated in FIG. 3 as the top of a second fictive pyramid which is restricted by an rectangular area A of the image when being displayed on the screen 10. Said position Q, that means the ideal position for the viewer, is reached when the second fictive pyramid is similar to the first pyramid.
More specifically, the first and the second pyramid are similar if the following two conditions are fulfilled simultaneously:
a) Q lies on a line L which is orthogonal to the area A of the displayed image and goes through the centre of A; and
b) the distance between Q and the centre of A is such that the top angles of the two pyramids are equal.
If condition a) is not fulfilled there will be an oblique second pyramid; if condition b) is not fulfilled there will be an erroneous perspective shortening in case of occulsion, i.e. different objects of the original 3D scene get false relative apparent depths. The case that the condition a) is not fulfilled is more annoying to the viewer than the case that condition b) is not fulfilled.
Expressed in other words, if the current position O of the viewer watching the image on the screen 10 does not correspond to the position Q, i.e. if there is a distance |Q-O| between said positions Q and O, the viewer will see a perspectively deformed image. A large distance |Q-O | can result in reduced visibility of the displayed image and worse in reduced readability of text.
In prior art a suboptimal approach is known to overcome these disadvantages by adapting the displayed image to the current position of the viewer. More specifically, that approach proposes to rotate the physical screen by hand or by an electric motor such that condition a) is fulfilled; e.g. Bang and Olufsen sells a TV having a motor for rotating the screen.
According to that approach rotation of the screen is controlled in response to the distance |0-Q | between the position O of the viewer and the fix position Q. Rotation of the screen by hand is inconvenient for the viewer and the rotation by motor is expensive and vulnerable. Moreover, condition b) can not be fulfilled by that approach.
Starting from that prior art it is the object of the invention to improve a method and apparatus for providing an image to be displayed on a screen such that the application of the method is more convenient to a user or a viewer of the image.
Said object is solved by the method according to claim 1 comprising the steps of estimating the current spatial position O of a viewer in relation to a fixed predetermined position Q representing a viewing point in front of the screen from which the image could be watched without any perspective deformation; and providing the image by applying a variable perspective transformation to an originally generated image in response to said estimated current position O of the viewer.
Advantageously said perspective transformation enables a viewer in any current spatial position O in front of the screen to watch the image on the screen without perspective deformations. Consequently, the visibility of the displayed image and in particular the readability of displayed text is improved.
Said transformation is convenient to the viewer because he does not get aware of an application of the transformation when he is watching the images on the screen. There is no physical movement of the screen like in the prior art.
Moreover, the implementation of the transformation can be realised cheaply and usually no maintenance is required.
The application of said method is in particular helpful for large screen TV""s or large monitors.
According to an embodiment of the invention the perspective transformation of the original image advantageously includes a rotation and/ or a translation of the co-ordinates of at least one pixel of the originally generated image. In that case an exact transformation of the position of the viewer into the ideal position Q can be achieved and perspective deformations can completely be deleted.
Preferably, the estimation of the position O of the viewer is done by tracking the head or the eye of the viewer.
Alternatively, said estimation is done by estimating the current position of a remote control used by the viewer for controlling the screen.
Preferably, the method steps of the method according to the invention are carried out in real-time because in that case the viewer is not optically disturbed when watching the image on the screen.
Further advantageous embodiments of the method according to the invention are subject matter of the dependent claims.
The object of the invention is further solved by an apparatus according to claim 12. The advantages of said apparatus correspond to the advantages outlined above with regard to the method of the invention.
Advantageously, the estimation unit and/ or the correcting unit is included in a TV set or alternatively in a computer. In these cases, there are no additional unit required which would otherwise have to be placed close to the TV set or to the computer.
The object of the invention is further solved by the subject matter of claim 16, the advantages of which correspond the advantages of the apparatus described above.